1. Field of the Invention
The present invention relates to toner for a two-component type developer used for electrophotography. More particularly, the present invention relates to toner, which does not include a charge control agent, suitably used in an electrophotographic image forming apparatus such as an electrostatic copying machine and a laser beam printer.
2. Description of the Related Art
A two-component type developer is used as one of the developers used for developing an electrostatic latent image on a photosensitive body in an electrophotographic image forming apparatus. The two-component type developer includes toner comprising a binder resin and a coloring agent such as carbon black, and magnetic carrier such as iron powder and ferrite particles.
An electrostatic latent image is developed by the following steps: the developer forms a magnetic brush shape on a developing roller by a magnetic field thereof and is carried out to the photosensitive body. In this step, the toner is charged by friction with the carrier so as to have a desired charge and polarity of charge. Then, the developer is contacted with the photosensitive body by the developing roller, resulting in attaching the toner onto the electrostatic latent image formed thereon. Generally, the toner includes a charge control agent which controls and stabilizes the charge of the toner so as to attach a constant amount of the toner on the electrostatic latent image and provide a good developed image for a long period of time. Negatively charged toner includes a negative charge control agent such as a dye of a metal complex including a metal ion such as chrome(III) (for example, an azo compound--chrome(III) complex), and an oxycarboxylic acid--metal complex (for example, a salicylic acid--metal complex) (Japanese Laid-Open Patent Publication No. 3-67268). Positively charged toner includes a positive charge control agent such as an oil soluble dye including nigrosine and an amine type charge control agent (Japanese Laid-Open Patent Publication No. 56-106249).
Many metal complexes, including a heavy metal ion such as a chrome ion, are used as a conventional charge control agent. They are carefully selected, in terms of environmental safety, so that only those having passed various toxicity tests and safety tests alone are used. Therefore, although they would be safe in themselves or when included in toner, it is more preferable to refrain from using the metal complexes including a heavy metal as the charge control agent. In addition, the charge control agent is expensive as compared with the other materials for toner such as a binder resin and a coloring agent, for example, carbon black. Therefore, although the charge control agent has a content of merely several %, this results in increasing the price of the resultant toner. Accordingly, it is desired to develop toner having no charge control agent of a metal complex.
Furthermore, when conventional toner is used for a long period of time, the toner components tend to attach on a surface of the carrier particle. The attached components are called a spent. The spent makes the carrier charge with the same polarity as the toner, resulting in the disadvantages that the toner can be scattered and transfer efficiency of toner image is decreased.